Formation of Co nanoclusters in epitaxial Ti_0.96Co _0.04O₂ thin films and their ferromagnetism

Anatase Ti_0.96Co_0.04O₂ films were grown epitaxially on SrTiO₃ (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, P_O2, during the growth was systematically varied. As P_O2 decreased, the growth behavior was changed from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like one, which resulted in an increase in the saturation magnetization. These structural and magnetic changes were explained in terms of the formation of cobalt clusters whose existence was proved by transmission-electron-microscope studies. Our work clearly indicates that the cobalt clustering will cause room-temperature ferromagnetism in the Co-doped TiO₂ films.